1. Field of the Invention
Aspects of the present application relate to a shading correction circuit which is used for correcting unevenness in luminance occurring in an image pickup element of a digital still camera or the like.
2. Description of Related Art
Generally, limb darkening occurs in a periphery of an image pickup frame in an image processing apparatus using an image pickup element including a CCD sensor, a COME sensor or the like. To correct such limb darkening, the image processing apparatus is equipped with a shading correction circuit. It is necessary not only to improve accuracy in correction of the shading correction circuit, but also to reduce the shading correction circuit size. For example, Japanese Patent Laid-Open Publication No. 2002-10122, Japanese Patent Laid-Open Publication No. 2002-218298, Japanese Patent Laid-Open Publication No. 2005-341033 and so on disclose the shading correction technique.
Japanese Patent Laid-Open Publication No. 2002-10122 discloses a shading correction system. In this system, an image of a uniform light source is taken to produce reference shading correction data and this reference shading correction data is stored in a memory. Next, when the shading correction is performed on a read out image, the reference shading correction data undergoes an appropriate correction according to image pickup magnification to perform the shading correction. However, it is difficult to achieve sufficient reduction in the memory capacity used for storing the reference shading correction data, in the above-described shading correction system.
Japanese Patent Laid-Open Publication No. 2002-218298 discloses an image pickup apparatus. In this apparatus, a shading correction coefficient corresponding to luminance data taken from respective pixels of the image pickup element is extracted from the shading correction coefficients stored in a memory. However, the above-described image pickup element requires a large capacity memory in order to store the shading correction coefficients corresponding to the respective pixels, in the memory.
Japanese Patent Laid-Open Publication No. 2005-341033 discloses another image pickup apparatus. In this apparatus, the shading correction is performed on a picked-up image with the use of a dedicated correction table with respect to each color component. This apparatus is, also, equipped with a shading correction table for a lens system and a shading correction table for a sensor system. However, this image pickup element also requires a large capacity memory in order to store the shading correction tables.